# Meridian Definitions for Land Surveyors

**meridian****—**A north-south reference line; a great circle through the geographical poles of the Earth. The term usually refers to the upper branch, i.e., that half from pole to pole which passes through a given place; the other half being called the lower branch.

**meridian, assumed—**A meridian chosen as a matter of convenience. See also *meridian, grid; azimuth, assumed.*

**meridian, astronomic–**A line on the surface of the Earth with the same astronomic longitude at every point. Because the deflection of the vertical is not the same at all points, an astronomic meridian is an irregular line, not lying in a single plane. The astronomic meridian and the line whose astronomic azimuth at every point is south or north (0° or 180^{0}) are not necessarily coincident, although in land surveying the term “astronomic meridian” is sometimes applied to the north-south line which has its initial point on a prescribed astronomic meridian.

**meridian, auxiliary guide **[USPLS]—A new guide meridian established, when required, for control purposes where the original guide meridians were placed at greater intervals than 24 miles. Auxiliary guide meridians may be required to limit errors of old surveys or to control new surveys; they are surveyed in all respects as are regular guide meridians.

**meridian, celestial****—**The hour circle which contains the zenith. Also defined as the vertical circle which contains the celestial pole. The plane of the celestial meridian is parallel with—but, because of the deflection of the plumb line, usually does not contain—the axis of rotation of the Earth. The intersection of the celestial meridian with the plane of II e horizon is the astronomic meridian line used in plane surveying.

**meridian, central-**^{1}The line of longitude at the center of a projection. Generally the basis for constructing the projection. ^{2} The longitude of origin at the center of each zone of the Universal Transverse Mercator (1.1131) grid. The central meridian is arbitrarily numbered 500,000 and is called a false casting. ^{3} The meridian used as the y-axis for computing projection tables for a state coordinate system. The central meridian of the system usually passes close to the center of the figure of the area or zone for which the tables are computed. See also *coordinates, grid.*

**meridian, convergence of—**See *convergence of meridians.*

**meridian, geodetic—**A line on an ellipsoid which has the same geodetic longitude at every point. If the ellipsoid is an ellipsoid of revolution. ; used for the purposes of triangulation, a geodetic meridian is an till’s whose plane contains the minor axis of the ellipsoid, and whose geodetic azimuth at every point is 0° (south) or 180° (north).

**meridian, grid—**A line through a point parallel to the central meridian or y-axis of a system of plane rectangular coordinates. See also *meridian **assumed; azimuth, assumed.*

**meridian, guide **[USPLS]—An auxiliary governing line projected north along an astronomical meridian from points established on the base** **line or a standard parallel, usually at intervals of 24 miles east or west o the principal meridian, on which township, section, and quarter-section corners are established.

**meridian, local (or reference)—**The meridian through any particular place or observer serving as the reference for local time, in contrast with Greenwich Meridian.

**meridian, magnetic—**The vertical plane in which a freely suspended, symmetrically magnetized needle not influenced by transient, artificial or magnetic disturbance will come to rest. Also, a curve on the Earth surface tangent to such a plane at each place it touches.

**meridian, prime—**The meridian of longitude 0°. used as the origin for the measurement of longitude. The meridian of Greenwich, England is almost universally used for this purpose. However, local or nations prime meridians are occasionally used.

**meridian, principal **[USPLS]—A line extending north and south along the astronomical meridian and passing through the initial point, along which township, section, and quarter-section corners are established. The principal meridian is the line from which the survey of the township boundaries is initiated along the parallels.

**meridian angle—**The arc at the celestial equator, or the angle at the celestial pole, between the upper branch of the local celestial meridian and the hour angle of the object of interest, measured eastward or westward from the local celestial meridian through 180° and labeled to indicate the direction in which the angle was taken.

**meridian distance-**^{1}The average distance of the ends of the line from the meridian. ^{2} The distance of the middle of a course from the meridian. ^{3} The perpendicular distance, in a horizontal plane, of a point from a meridian of reference. Also called “meridional distance.”

**meridian passage—**See *transit *(definition **1**).

**meridian plane-**^{1}A plane containing the least axis of an ellipsoid representing the Earth; the plane is usually taken to be parallel to the Earth’s axis of rotation. ^{2} A plane containing the normal at a point on an ellipsoid representing the Earth, and parallel to the least axis of the ellipsoid. ^{3} A plane containing a meridian on the celestial sphere.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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